Recently, the recording density of the optical disc media has become increasingly higher. In general, a recordable optical disc medium has track grooves therein in advance, and information is recorded along the track grooves, i.e., on the track grooves or an area interposed between the track grooves (referred to as a “land”). The track grooves are sine-like wobbles, and the information is recorded in synchronization with clocks generated based on the period of the wobbles. Addresses are provided along the track groove in order to record information at prescribed positions on a recording face of the optical disc medium. Three exemplary structures for providing addresses will be described below.
(1) Japanese Laid-Open Publication No. 6-309672 discloses an optical disc in which wobbled track grooves are formed locally and intermittently and address information can be reproduced as so-called pre-pits. In this case, an address-only area and a data-only area for recording information exist on the track groove.
(2) Japanese Laid-Open Publication No. 5-189934 discloses an optical disc in which frequency-modulated wobbles are provided and address information (sub information) is recorded using the frequency of the wobbles. In this case, data information is overwritten on the address information.
(3) Japanese Laid-Open Publication No. 9-326138 discloses an optical disc in which pre-pits are formed between adjacent track grooves and addresses are formed by the pre-pits.
In consideration of the higher density recording which will be required in the future, all the above-mentioned structures have their own problems.
In the structure of (1), the space for data is reduced by the space required for the addresses (so-called “overhead”). Thus, the memory capacity is inevitably reduced by the space for the addresses.
The structure of (2) has the following problem. The wobbles are originally provided mainly for the purpose of generating clocks for recording information, and thus are preferably formed with a single frequency. When the wobbles are formed with a single frequency, highly precise recording clock signals can be generated merely by multiplying and synchronizing a wobble reproduction signal using a PLL or the like. When the wobbles have a plurality of frequency components, however, the frequency band to which the PLL can adapt itself needs to be reduced relative to the case where the wobbles have a single frequency in order to avoid pseudo lock of the PLL. Then, it may undesirably occur that the PLL cannot sufficiently follow disc motor jitters or jitters generated by, for example, de-centering of the disc. This results in jitters remaining in a recording signal.
In the case where a recording film formed on a recording face of an optical disc is a phase change film, the S/N ratio of the recording film can undesirably be reduced as rewriting is repeated. Even when this occurs, wobbles with a single frequency allow the noise component to be removed using a bandpass filter for a narrow band. However, when the wobbles are frequency-modulated, the band to be passed needs to be enlarged in order to allow for the modulated frequencies. As a result, the noise component is mixed in with a wobble reproduction signal and thus further increases the jitters. Such an increase of jitters is not preferable since the jitter margin is decreased as the recording density is increased.
In the structure of (3), the pre-pits naturally influence reading of the information stored in the adjacent track grooves. Thus, it is difficult to provide a sufficient number of pre-pits each having a sufficient length. Therefore, there is an undesirable possibility that the number of detection errors is increased especially when the recording density is significantly high.
In light of the above-described problems, the present invention has an objective of providing an optical disc medium for minimizing overhead and describing addresses with wobbles having a single frequency, an optical disc apparatus and an optical disc reproduction method for reproducing the optical disc medium.